International Journal of Marine Science, 2024, Vol.14, No.3, 231-244 http://www.aquapublisher.com/index.php/ijms 234 Figure 1 Photos, taken along a pCO2/pH gradient in Papua New Guinea (year 2014), and text depict three potential scenarios for coral reefs under present day and future (year 2050 and 2100) OA conditions if we continue on current predicted CO2 emissions trajectories (Adopted from O'Brien et al., 2016) Image caption: Present Day illustrates a healthy coral reef with high structural complexity and diversity that is likely colonized by beneficial microbial associates and experiences a low incidence of disease. As pCO2 increases, scenarios for years 2050 and 2100 illustrate the successional degradation of coral reef heterogeneity (structural diversity), destabilization of microbial associations and increase in disease, transitioning to an alternative stable state composed of competitive dominants (e.g., sponges, macroalgae) and/or sediment/rubble. Arrows from green to red indicate the positive to negative changes a coral reef might experience over time (Adopted from O'Brien et al., 2016). 4 Ecosystem-Level Impacts 4.1 Changes in biodiversity Ocean acidification has profound implications for marine ecosystems, influencing biodiversity, food web dynamics, and ecosystem services. This section explores these impacts in detail. Ocean acidification leads to significant changes in marine biodiversity, primarily by affecting species differently based on their physiological and ecological characteristics. Calcifying organisms, such as corals, mollusks, and certain plankton, are particularly vulnerable due to their reliance on calcium carbonate for their skeletal structures. The reduction in pH decreases the availability of carbonate ions, essential for calcification, leading to weaker shells and skeletons. This vulnerability can result in decreased biodiversity as these organisms struggle to survive and reproduce under more acidic conditions (Zunino et al., 2021). Additionally, acidification can alter species composition and abundance, favoring non-calcifying over calcifying species, and causing shifts in community structure. Studies at CO2 seeps have shown that areas with lower pH levels have reduced species diversity and are dominated by algae and non-calcifying organisms (Hall-Spencer and Harvey, 2019) (Figure 2). Figure 2 illustrates the ecosystem properties, functions, and services provided by coastal habitat-forming species and the communities they support. The central part of the diagram shows temperate and tropical habitat-forming species, such as coral reefs and seagrass beds, which provide the foundational structure for ecosystems. Surrounding this central part is a layer labeled "Species," indicating how these habitat-forming species support various ecosystem communities. The outermost layer is divided into three sections: ecosystem properties, ecosystem functions, and ecosystem services. Ecosystem properties include habitat complexity, species richness, and coverage. Ecosystem functions encompass productivity, biodiversity, and nutrient cycling. Ecosystem services involve food provision, recreation, and coastal protection. These properties, functions, and services are interconnected and collectively maintain the health and stability of the ecosystem. Arrows within the diagram
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